In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to form characters or images. The ink-jet printing methods have become rapidly spread because of various advantages such as easiness of full coloration, low cost, capability of using a plain paper as the recording medium, non-contact with printed characters or images, etc.
In recent years, in order to impart good weathering resistance and good water resistance to printed materials, an ink containing a pigment as a colorant has been extensively used.
On the other hand, it has been required to print characters or images on recording media for commercial printing purposes using a low-liquid absorbing coated paper such as an offset coated paper, or a non-liquid absorbing resin film such as a polyvinyl chloride resin film, a polypropylene resin film and a polyester resin film.
It is known that when characters or images are printed on the low-liquid absorbing or non-liquid absorbing recording media by the ink-jet printing methods, there tend to occur problems such as prolonged drying time owing to slow absorption or no absorption of liquid components, and deterioration in quick-drying fixing properties of the ink and image uniformity upon printing the characters or images, etc., thereon.
JP 2013-107951A discloses an aqueous pigment ink set constituted of a magenta ink, a cyan ink, a yellow ink and a black ink, in which the respective inks include at least water, a solvent and a pigment, and the pigment includes a specific pigment. In JP 2013-107951A, it is described that the ink set is excellent in effects of suppressing strike-through of the ink when printed on an offset-printing wood-free paper and preventing occurrence of color unevenness of printed characters or images on a coated paper.
JP 2003-82264 A discloses an aqueous pigment ink for ink-jet printing which includes carbon black and/or an organic pigment, glycerol, triethylene glycol monobutyl ether and a specific glycol-based ether. In JP 2003-82264A, it is described that the ink is free from separation of triethylene glycol monobutyl ether from the pigment component and glycerol even when water contained in the ink is completely evaporated therefrom, hardly suffers from formation of aggregated products, and is excellent in ejection properties and can be prevented from occurrence of clogging of the ink at a tip end of an ink-jet print head even when used for ink-jet printing.